Abstract:We investigated effects of testosterone treatment on timing and rate of moult of wing-feathers in male Shelducks Tadorna tadorna, and whether this treatment influences the timing of moult in untreated females. Three experimental treatments were conducted, each with two groups of sexual inactive male Shelducks, which were kept under natural photoperiods in The Netherlands. Each group consisted of two pairs and a single male. Males received implants with testosterone-proprionate for seven weeks shortly before the natural moult onset of wing-feathers. Start of treatment was 27 June (treatment I), 5 July (treatment II), and 18 July (treatment III). Males delayed their moult until the end of hormone treatment. The degree of delay in start of moult within the same treatment was related to androgen blood levels. The untreated females of the testosterone-treated males delayed also their onset of moult. The timing of the end of moult was unaffected by the hormonal treatment. Males that moulted late did not speed up the rate of moult, or changed the pattern of moult, but some of these birds stopped moult before completion. The data indicate that under natural photoperiods the statt of moult is quite flexible, requires low levels of androgens, and is under the influence of social factors, while the end of moult is fixed.